This invention relates to thermoplastic shipping bags and, particularly, to said bags for heavy duty use having one or more plies of a uni-axially oriented cross-laminated polyethylene.
Thermoplastic shipping bags are an economical means for the packaging, transportation and storage of a wide variety of products in granular, bead, pellet or powder form. These bags may be either open ended, requiring separate provision for closing, or fitted with a valved opening. The most commonly used type of thermoplastic bags are manufactured from film made by blown film processes using low density polyethylene and rubber modified high density polyethylene resins and coextruded versions of these resins and allied copolymers. The film may be subsequently converted to bags by a variety of sealing techniques. Some of these simple thermoplastic films may be converted to bags by sewing but this has the disadvantage of badly weakening the films and introducing unwanted holes in the bag through which moisture or other contaminants can enter or allow environmental contaminants to exit. Other bags may be manufactured by the multiple folding and glueing of the bag ends, but this has the serious disadvantage of requiring very expensive converting equipment, as well as the use of expensive and difficult to control two component glue systems.
Because of these drawbacks the use of heat sealing is the desired method of manufacture of thermoplastic bags.
At the present, heat sealed thermoplastic bags are commonly used for the packaging of low cost, non-hazardous products. Their use for higher cost and hazardous products, however, has been badly restricted because of the poor puncture and snag resistance of the bags compared with multi-wall paper or woven plastic bags of related composition structures.
It is known in the art that the required level of puncture and snag resistance can be built into a thermoplastic film manufactured from linear ethylene polymers such as high density polyethylene or linear low density polyethylene, by uni-axially cold drawing such film and subsequently laminating two of these layers in such a manner that the draw-induced orientation of one web typically is 90.degree. to that of the other web. Thus, the uni-axially oriented sheets of polyethylene are laminated one to the other in such a manner that the directions of orientation cross each other. The resulting composite laminated film, generally known as a cross-plastic, cross-laminate, or cross-laminate ply has high puncture and snag resistance compared to an equivalent gauge non-oriented blown film. Films of this type have thus been used for sewn and glued shipping bags for high value and hazardous materials.
However, these materials have one serious drawback in that they cannot reliably be heat sealed on conventional bag making machinery to provide bags for heavy duty use. This has seriously impeded their utility and acceptability in the market place notwithstanding the aforementioned advantages of heat sealed bags.